Final answer:
The M checkpoint occurs near the end of the metaphase stage of mitosis and ensures proper attachment of sister chromatids to the spindle microtubules.
Step-by-step explanation:
The M checkpoint, also known as the spindle checkpoint, occurs near the end of the metaphase stage of mitosis. It is responsible for determining if all the sister chromatids are correctly attached to the spindle microtubules. The cycle will not proceed until the kinetochores of each pair of sister chromatids are firmly anchored to spindle fibers arising from opposite poles of the cell.